[suggestion] add initial gap/offset in LineMaterial example

[lslzl3000]

The LineMaterial example is a very good start to use in many cases. However, I think we need another gapoffset<float> parameter to support dash animation.

Having this property is what allows to have a dash animation because you can just tween this value and doing so build the effect of the dashes moving. It will be a very useful parameter.

Hope someone could dig current code, add this feature in shaders😜